Actually that page is even more confusing, as it talks about "processing times" as well.

That page is consistent with msvc implementation. OTOH glibc's implementation is weird for me.

Code:

/***
*clock_t clock() - Return the processor time used by this process.
*
*Purpose:
* This routine calculates how much time the calling process
* has used. At startup time, startup calls __inittime which stores
* the initial time. The clock routine calculates the difference
* between the current time and the initial time.
*
* Clock must reference _cinitime so that _cinitim.asm gets linked in.
* That routine, in turn, puts __inittime in the startup initialization
* routine table.
*
*Entry:
* No parameters.
* itime is a static structure of type timeb.
*
*Exit:
* If successful, clock returns the number of CLK_TCKs (milliseconds)
* that have elapsed. If unsuccessful, clock returns -1.
*
*Exceptions:
* None.
*
*******************************************************************************/